Corrosion of metals is an extremely important field with great economic and engineering implications at the Oak Ridge Y-12 Plant. To effectively combat corrosion, one must understand the processes occurring. This paper shows the utility of Secondary Ion Mass Spectrometry (SIMS) data for elucidating the processes occurring in one particular corrosion process - the oxidation of uranium by water - and for validating a theoretical model. It had long been known that the oxidation of uranium by water is retarded by the presence of oxygen gas and the retardation has been assumed to occur by site blocking at the surface. However, when alternate isotopic exposures were made, followed by exposure to a mixture of /sup 16/O/sub 2/ and /sup 18/OH/sub 2/, the rapid exchange of /sup 16/O and /sup 18/O occurred in the oxide layer, but the further oxidation by water in this and subsequent exposures was retarded for up to 21 hours. This shows graphically that OH/sub 2/ is not held up at the surface and that the retarding mechanism is effective at the oxide/metal interface rather than at the surface. The effectiveness of the O/sub 2/ to retard the further water oxidation was much reduced if no water-formed …